Observation of a bursty high-frequency edge instability in DIII-D strongly shaped divertered negative triangularity plasmas*

During the 2023 DIII-D Negative Triangularity (NT) campaign, nearly all discharges in the diverted “baseline” NT shape ( experienced an edge coherent mode as observed by the magnetics with a mode frequency of 3-10 kHz and  toroidal mode number. The instability is cyclic, with a “burst” frequency of 100-300 Hz. The burst onset drives a reduction in the edge electron temperature (Te) as measured by ECE and ECEI, followed immediately by a recovery period in Te. ECE and BES data show the mode to be radially localized to the outer 10% of toroidal flux, while ECEI shows a possible external mode with the radial mode structure extending beyond the seperatix. Experimental analysis in combination with stability modelling is used to identify the mode. GATO and ELITE are utilized to assess the ideal stability of the discharges, while linear M3D-C1 provides an initial look at non-ideal effects. The primary goal of this work is to identify the type of instability observed in the DIII-D NT experiments, as well as to gain insight into the bursting nature of the instability. Significant work is still needed to fully understand these negative triangularity edge modes, but doing so may better help us understand the unique confinement characteristics of the NT configuration.